Distinct qualitative differences in microbial populations present in subgingival plaque samples were seen using phase-contrast microscopy in a) subjects with excellent periodontal health, b) subjects with marginal gingivitis and no pocket formation, and c) subjects with destructive periodontitis. The microbiological status observed in periodontal health characteristically presented with loosely organized "thread" froms coated with small cocoidal forms. Spirochetes and other large motile forms were ever seen and few white blood cells were observed. In non-destructive marginal gingivitis, denser masses of thread forms and cocci were seen, as well as, occasional clusters of unorganized spirochetes and motile rods. White blood cells were observed, but only in the range of + 100 per field. In persons with destructive periodontitis, large populations of motile bacteria and white blood cells in a too-numerous-count range were noted, in addition to large static masses of filaments, rods and cocci. The motile forms consisted of spirochetes, various rods and amoebae. Based on our findings, we suggest that both diagnosis and management of periodontal lesions can be improved by use of phase contrast microscopy to monitor the microbiological status of diseased sites.